1. Technical Field
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device in which an ambient light photosensor that senses external light is built into the liquid crystal display panel.
2. Related Art
Recent years have seen thin-type display panels come into use in telecommunications equipment and electronic equipment in general. Among such panels, liquid crystal display panels are the most widely used. In liquid crystal display panels the liquid crystals are non light-emitting, which means that the images displayed are hard to see in dark places. Therefore these panels are provided with a backlight or sidelight (“backlight” below), and when the external light is dim, the backlight is lit in order to render visible the images displayed.
However, with manual control one must perform manipulations to turn the backlight on or off according to the brightness of the external light. Such manipulations are bothersome, and furthermore when it is bright one sometimes ends up turning the backlight on unnecessarily, which wastefully increases the power consumption and, where the display is used in a mobile telephone or similar equipment, could cause the battery to run down quickly.
Accordingly, in order to eliminate such inconvenience, technology has been developed whereby an ambient light photosensor is built into the liquid crystal display panel, the brightness of the external light is sensed by the ambient light photosensor, and on/off switching of the backlight is controlled according to the sensing results (see JP-A-2002-131719, JP-A-2000-122575 and JP-A-11-84426).
For instance, the liquid crystal display device set forth in JP-A-2002-131719 uses as the ambient light photosensor a thin film transistor (TFT), which is fabricated simultaneously with the TFTs used as the liquid crystal display panel's switching elements, while the liquid crystal display device set forth in JP-A-2000-122575 deploys an external light illuminance sensor and a backlight illuminance sensor on the panel circuit board, controlling the backlight according to the results of sensing by both sensors, and furthermore the liquid crystal display device set forth in JP-A-11-84426 deploys the ambient light photosensor in a place distanced from the drive circuits around the liquid crystal's periphery and from the external terminals, so that the sensor is not affected by high frequency noise generated by, or heat emitted from, those drive circuits, etc.
The TFT ambient light photosensor has the characteristic that when no light is shining on it a slight leakage current (dark current) flows in its gate-OFF region, and when light shines on it a current (leakage current) flows that is of size corresponding to the intensity (brightness) of the light. Since the TFT ambient light photosensor utilizes this leakage current to sense the brightness of the external light, and the leakage current is extremely feeble, the output from the TFT ambient light photosensor is susceptible to the influence of external noise.
Therefore, if the aforesaid ambient light photosensor is built into one of the liquid crystal display panel's substrates, for example into the active matrix substrate (also called the array substrate), then since there are peripheral drive circuits for driving the liquid crystal, as well as external terminals, etc., deployed on the active matrix substrate, high-frequency signals and heat emissions, etc., from such circuits, etc., may affect the sensor. To counter this, the liquid crystal display device of JP-A-11-84426 places the ambient light photosensor in a location distanced from the peripheral drive circuits and external terminals that generate high-frequency signals and heat emissions, so that it is not affected by such noise. However, there are lead-out lines, including at least a power line and an output line, coupled to the TFT ambient light photosensor, and these lead-out lines are passed around the periphery of the display section. Consequently these lead lines, particularly the output line, will be susceptible to the influence of external noise. Moreover, since parasitic capacitance is generated between the output line and the common electrode on the opposed substrate, there is danger that the output line will be affected by the common electrode voltage (below, “VCOM voltage”) that is applied to the common electrode. Also, the ambient light photosensor may be directly affected by the VCOM voltage.
Ambient light photosensors of this kind are known in which, because of the susceptibility to external noise, electrostatic shielding film is provided on the active matrix substrate (see JP-A-1-143257 and JP-A-5-95100). But with such ambient light photosensors a conductive film and insulator for electrostatic shielding are provided between the active matrix substrate and the gate wiring, and providing such conductive film entails a special process in addition to the usual fabrication processes for forming the ambient light photosensor on the active matrix substrate. This makes the fabrication troublesome and leads to a sharp rise in overall cost.